Fix ProcessExecutor bootstrapping in monorepo-helper

Without this fix Composer throws a LogicException with the message: "You
must use the ProcessExecutor instance which is part of a Composer\Loop
instance to be able to run async processes.", but only if the
monorepo-helper is not run on the `master` or `main` branch. The
inspiration for this fix was taken from the resolving commit of
composer/composer issue #10703.

See: composer/composer#10703

Change-Id: I0af1c8f8f5badb0594c978a9c5dc8361e7bd6b81
Issue-Link: https://project.pronovix.net/issues/18528

src/Composer/Plugin.php

@@ -33,7 +33,6 @@
 use Composer\Plugin\PluginEvents;
 use Composer\Plugin\PluginInterface;
 use Composer\Util\Filesystem as ComposerFilesystem;
-use Composer\Util\ProcessExecutor;
 
 /**
  * Monorepo Helper plugin definition.
@@ -74,7 +73,7 @@ public function activate(Composer $composer, IOInterface $io): void
             return;
         }
 
-        $process = new ProcessExecutor($io);
+        $process = $composer->getLoop()->getProcessExecutor();
         $monorepoRoot = $configuration->getForcedMonorepoRoot();
         $output = '';